Determination Of Background Concentrations Research Articles

Microfabricated Collector-Generator Electrode Sensor for Measuring Absolute pH and Oxygen Concentrations.

Fast-scan cyclic voltammetry (FSCV) has attracted attention for studying in vivo neurotransmission due to its subsecond temporal resolution, selectivity, and sensitivity. Traditional FSCV measurements use background subtraction to isolate changes in the local electrochemical environment, providing detailed information on fluctuations in the concentration of electroactive species. This background subtraction removes information about constant or slowly changing concentrations. However, determination of background concentrations is still important for understanding functioning brain tissue. For example, neural activity is known to consume oxygen and produce carbon dioxide which affects local levels of oxygen and pH. Here, we present a microfabricated microelectrode array which uses FSCV to detect the absolute levels of oxygen and pH in vitro. The sensor is a collector-generator electrode array with carbon microelectrodes spaced 5 μm apart. In this work, a periodic potential step is applied at the generator producing transient local changes in the electrochemical environment. The collector electrode continuously performs FSCV enabling these induced changes in concentration to be recorded with the sensitivity and selectivity of FSCV. A negative potential step applied at the generator produces a transient local pH shift at the collector. The generator-induced pH signal is detected using FSCV at the collector and correlated to absolute solution pH by postcalibration of the anodic peak position. In addition, in oxygenated solutions a negative potential step at the generator produces hydrogen peroxide by reducing oxygen. Hydrogen peroxide is detected with FSCV at the collector electrode, and the magnitude of the oxidative peak is proportional to absolute oxygen concentrations. Oxygen interference on the pH signal is minimal and can be accounted for with a postcalibration.

Gas-Chromatographic Determination of Gaseous Hydrocarbons in Aqueous Solutions Using Chromatomembrane Gas Extraction

A procedure was developed for the gas-chromatographic determination of gaseous hydrocarbons (C1–C4) in aqueous solutions using the continuous chromatomembrane gas extraction of test substances from a flow of the test solution. The procedure provides the rapid determination of background concentrations of methane in natural waters (0.1–1.0 μg/L).

Acetone and acetonitrile in the tropical Indian Ocean boundary layer and free troposphere: Aircraft‐based intercomparison of AP‐CIMS and PTR‐MS measurements

We have performed aircraft‐borne intercomparison measurements of acetone and acetonitrile using two different mass spectrometric techniques: A Proton Transfer Reaction Mass Spectrometer (PTR‐MS) operated and developed by the Institute of Ion Physics, University of Innsbruck, and an Atmospheric Pressure Chemical lonization Mass Spectrometer (AP‐CIMS) operated and developed by the Max Planck Institute for Chemistry, Mainz. The two devices were employed for atmospheric trace gas measurements over the Indian Ocean on board the National Center for Atmospheric Research C‐130 research aircraft during the Indian Ocean Experiment (INDOEX) intensive field phase in February and March 1999. Both instruments use reactions of ions with atmospheric trace gas molecules and mass spectrometric detection techniques for the determination of trace gas concentrations. For the detection of acetone and acetonitrile, both mass spectrometers operate in the positive ion mode and detect the protonated species of these compounds. One difference of the two systems is the operating pressure of the reaction chamber, where the ion molecule reactions take place. While the PTR‐MS uses a low‐pressure (2.2 hPa) drift tube, a much higher pressure of 300 hPa is used by the AP‐CIMS. Other differences are the determination of background concentrations and the calibration procedure. The AP‐CIMS uses a charcoal filter, and the PTR‐MS uses a Pt catalyst. For the AP‐CIMS an on‐line calibration is necessary, which is performed during the flights. The intercomparison measurements described here were carried out on two research flights along a north‐south transect along 73°E between 6°N and 8°S. Altitudes between 100 m and 6500 m above sea level were covered. Backward trajectory analyses indicate that the sampled air was taken from polluted air masses originating from India and the Bay of Bengal as well as from remote southern hemispheric air. During the two flights on February 20 and 24 the position of the Intertropical Convergence Zone was located around 1.50S in the investigated area. The measurements of both instruments of acetone and acetonitrile indicate a strong gradient in the boundary layer and a weaker one in the free troposphere. The values measured by the PTR‐MS are higher than the ones by the AP‐CIMS, but are within the combined error range of ±20–30% for each instrument. Acetone mixing ratios up to 2.5 parts per billion (ppb) and acetonitrile up to 0.4 ppb were measured in the northern hemispheric boundary layer. The measurements in the free troposphere give values between 0.3 ppb and 1.0 ppb for acetone and 0.12 ppb and 0.3 ppb for acetonitrile.

Determination of Background Concentrations of Inorganics in Soils and Sediments at Hazardous Waste Sites

Hazardous waste sites may pose a threat to human health and the environment when toxic substances are released. However, the contaminants present at a waste site may have originated ‘on-site’ (i.e., resulting from releases attributable to site activities) or ‘off-site’ (i.e., resulting from sources not on-site). Off-site substances may result either from natural sources (e.g., erosion of naturally occurring mineral deposits) or anthropogenic sources (e.g., widespread contamination from automobile exhaust in urban areas). To determine the appropriate action to take at a hazardous waste site, the U.S. Environmental Protection Agency (EPA) must distinguish between substances directly attributable to the hazardous waste site (i.e., site contaminants) and those attributable to natural background concentrations. The most important factor to consider when determining background concentrations is to ensure that the physical, chemical, and biological aspects of the media to be sampled at both the contaminated site and the background site are as similar as possible. Inorganics, in particular metals, are addressed. Radionuclides are not specifically addressed; however, metals with radioactive isotopes that may be encountered at hazardous waste sites are included. There are references and data included in this paper that provide average concentrations and reference values for selected soils and sediments in the United States. Suggested sampling and monitoring design approaches that could be used by scientists and engineers faced with how to determine background concentrations are identified. The issues discussed include the selection of background sampling locations, considerations in the selection of sampling procedures, and statistical analyses for determining whether contaminant levels are significantly different on a potential waste site compared with a background site.